Civil wars are inherently complex and often feature a myriad of actors, whose interactions influence the intensity, duration and outcome of the conflict. The larger the number of actors involved in a conflict, the more complex it gets. While civil wars are often portrayed as a dyadic interaction between the government and a single rebel group, this is far from the reality. Between 1946 and 2015, more than half of those countries that experienced civil wars saw two or more active rebel groups. Understanding multiparty conflicts better is important, as they are deadlier, more difficult to solve and more dangerous for civilians. This dissertation studies the causes and consequences of multiparty civil wars. It suggests that all actors in a conflict system with several actors influence each other, which impacts conflict dynamics. Four essays shed light on different aspects of these civil wars. Essay I studies the differences in formation rates of rebel groups across the states of Northeast India. It finds that potential rebel groups will only form when rebellion is perceived as a legitimate way to address grievances and when competition from already existing groups is not too high. Essay II looks at rebel group splintering: It focusses on relationships within rebel groups and finds that both vertical and horizontal relations affect the likelihood of splintering. Essay III studies violent interactions between rebel groups and investigates how different conflict dynamics influence interrebel fighting. It demonstrates that interrebel fighting is more likely when one of the rebel groups is more successful against the government and when negotiations are ongoing. Finally, Essay IV widens the scope of conflict actors by studying why rebels decide to fight against UN peacekeeping operations. It shows that only relatively strong rebel groups are likely to attack blue helmets. Taken together, this dissertation furthers our understanding of the causes and consequences of multiparty civil wars. It highlights the intricate web of relations that form between actors and that influence civil war dynamics. These relations matter not only for studying civil wars, but also for preparing negotiations or planning a peacekeeping mission.